PTQ Q1 2024 Issue

Comparison table – pressurising to different levels

Scenario 1 1st Cycle

Scenario 2

Scenario 3

1st Cycle

2nd Cycle

1st Cycle

2nd Cycle

Cn = mol% oxygen in nitrogen

0.10

0.10 4.28 2.19 1.50 3.00 1

0.10

0.10 7.07 3.58 1.50

Ci = mol% oxygen initially in space to be purged Cf = mol% oxygen finally in space to be purged

21.00

2.42 1.00 9.00

4.28 1.00 5.00

7.07 1.00 3.00

Pi = Initial pressure in bara Pf = Final pressure in bara

3.0

n = Number of pressure/de-pressure cycles Required nitrogen volume(Nm 3 ) [vessel volume -V ]

8 V 8 V

4 V

2 V

Total nitrogen volume for scenario (Nm³)

6 V

4 V

Note 1: Pressurisation is limited to 3 bara as oxygen content was below the target value of 4 vol%.

Table 4

Typically, hydroprocessing units undergo leak testing at varying pressure levels using nitrogen. For the context of this article, 20 barg is assumed as the maximum leak test nitrogen pressure. Upon successful completion of this test, a nitrogen circulation at 20 barg is established. The process involves an initial cycle for detecting and rectify- ing leakages. Then, a second cycle is used to stabilise the nitrogen circulation. Under conditions where the HP loop is successfully tested at 5 barg and pressurised in the first cycle from 5 barg to 20 barg, the total nitrogen requirement amounts to 35,000 Nm 3 , which includes leak test, inertising and pressurisation. Frequently, hydroprocessing licensors specify a nitrogen amount with a peak flow rate of 3,000 Nm3 /h. The stand- ard configuration for the nitrogen line to the HP loop is 3in, equipped with a full-bore globe valve. As per industry standards, the allowed depressurisation rate stands at 20 bar/hour, prompting operators to fully open the 3in globe valve. With a 3in API 600 type globe valve with a C v of 106 for a 1,500 lb rating, the flow results in 12,000 Nm3 /h. In contrast, when considering compressible flow through a 3in 80 sch. pipe, the nitrogen flow rate is 8,000 Nm³/h where upstream nitrogen pressure is at 8 barg and down- stream at atmospheric pressure. Key takeaway: The start-up nitrogen line for reactor loop pressurisation should be sized to match licensor-prescribed nitrogen demand in the utility summary. A noteworthy observation is that the peak nitrogen demand of a hydroprocessing unit significantly influences the overall peak demand of the refinery, especially when the refinery has multiple hydroprocessing units. At the design stage, it is proposed by some refiners that the peak nitrogen demand for hydroprocessing units be met using a tanker connected directly at the unit level. By this method, the hydroprocessing units’ peak nitrogen demand is removed from the overall refinery nitrogen balance. Consequently, potential savings are realised, as the requirement for a nitrogen liquid tank and vapouriser is eliminated. However, practical implementation can pose challenges due to lim- ited access during unit shutdowns, especially during cat- alyst loading/unloading. A prudent approach is to inertise one hydroprocessing unit at a time. To minimise risk, at present, hydroprocessing units are

not practising vacuum pulling while inertising the HP loop. This precaution helps to prevent potential hazards, such as oxygen entering the system due to leaks. If oxygen does enter, it could react with hydrocarbons and catalysts in the system, as well as compromise the iron sulphide passiva- tion layer. Key takeaway: For optimal refinery operations, inertise one hydroprocessing unit at a time, considering a minimum peak demand rate of more than 3,000 Nm 3 /h for a 3in nitro- gen line. As discussed in the previous section, hydroprocessing units are identified as one of the major units demanding peak nitrogen during start-up. Because of this, the hydropro- cessing unit’s LP section is commonly inertised with steam. Once sufficient steam out is completed, fuel gas is intro - duced after steam venting to the atmosphere is boxed up. The LP section is pressurised to the maximum fuel gas pres- sure, which is typically around 4 barg. However, it should be noted that if the normal pressure of the stripper exceeds, say, 10 barg, the provision for nitrogen pressurisation to the LP section is considered prudent since the nitrogen header pressure is approximately 8 barg. Such nitrogen pressurisa- tion is found essential during the start-up phase, especially when the stripper requires pressurisation beyond 4 barg. Key takeaway: It is recommended to provide a nitrogen con- nection in addition to a fuel gas connection for the LP section.  Start-up nitrogen for hydroprocessing units low-pressure (LP) section  Gap control for blanketing gas for vessel Normally, the surge drum level is permitted to fluctuate within a small band, typically within 5%, instead of main- taining strict control. Due to this small fluctuation in level, a swing in the drum pressure is caused. This pressure swing causes the blanketing valve to open when the level drops and the vent to flare to open when the level rises. The con - tinuous activation of these valves leads to a loss of nitrogen from the surge drum. Typically, a dead band is incorporated into the surge drum pressure controller output, allowing the pressure to fluctuate without opening the valves in the nitrogen and flare lines for minor demands. Key takeaway: By implementing this gap control for all

PTQ Q1 2024